Keratins are a family of proteins found in the hair, skin, and other tissues of vertebrates. Hair is a unique source of human keratins because it is one of the few human tissues that are readily available and inexpensive. Although other sources of keratins are acceptable feedstocks for the present invention (e.g. wool, fur, horns, hooves, beaks, feathers, scales, and the like), human hair is preferred because of its biocompatibility in human medical applications.
Keratins can be extracted from human hair fibers by oxidation or reduction using methods that have been widely published in the art. If one employs a reductive treatment, the resulting keratins are referred to as kerateines. If an oxidative treatment is used, the resulting keratins are referred to as keratoses. These methods typically employ a two-step process whereby the crosslinked structure of keratins is broken down by either oxidation or reduction. In these reactions, the disulfide bonds in cystine amino acid residues are cleaved, rendering the keratins soluble without appreciable disruption of amide bonds. Many of the keratins can remain trapped within the cuticle's protective structure, so a second-step using a denaturing solution is typically employed to effect efficient extraction of the cortical proteins (alternatively, in the case of oxidation reactions, these steps can be combined). This step has also been widely published in the art as solutions such as urea, transition metal hydroxides, surfactant solutions, and combinations thereof have been employed. Common methods employ the use of aqueous solutions of tris(hydroxymethyl) aminomethane in concentrations between 0.1 and 1.0M, and urea solutions between 0.1 and 10M.
When oxidation is selected as the extraction method of choice, strong oxidants are used to cleave the cystine amino acid and solubilize the keratin proteins. A preferred oxidant is peracetic acid. Peracetic acid (CH3COOOH) hydrolyzes into acetic acid (CH3COOH) and hydrogen peroxide (H2O2). It also undergoes homolysis to produce peroxyl (CH3COO−; CH3COOO−), hydrogen (H+), and hydroxyl (HO−) radicals. Hydroxyl radicals are very strong oxidizing agents due to their high standard reduction potential (2310 mV). When reacted with HO−, proteins decompose into fragments with carbonyl groups (—C═O) in the presence of oxygen (O2) and a small fraction forms protein aggregates via cross-linking. Both of these degraded and cross-linked forms are observed in keratose samples. Aside from oxidation of cystine, peracetic acid (most likely through the action of HO− and H2O2)) also reacts and modifies other amino acids of the protein chain. The free thiols (—SH) of cysteines are converted to sulfenic acid (—SOH), which are further oxidized into sulfinic (—SO2H) and sulfonic acid derivatives.
The ability to form a polymerized hydrogel is an important feature in biomaterials used as scaffolds for cells, agents for drug delivery or constructs to promote cell infiltration and tissue remodeling. Hydration of lyophilized keratose materials generally yields the formation of an elastic solid-like hydrogel at high solute concentrations (200 mg/ml in PBS). Rheological properties of these gels as well as their chemistries indicate that the primary mechanism of gelation is through polymer chain entanglement. Oxidation of free thiols eliminates the ability of oxidized keratins to reassemble via covalent disulfide bonding. Instead, other gelation determinant factors may include electrostatic and hydrophobic interaction. Keratin multimers may form a larger network through electrostatic attraction as suggested in the assembly of intermediate filament molecules in which the head (positive) and the tail (negative) domains of dimers potentially associate to form a tetramer. The negatively-charged sulfonic acid groups can also interact with the basic amino acid residues such as lysine, arginine, and histidine that escaped oxidation. Additionally, the coil regions of keratins that are rich in hydrophobic sequences may aggregate together to increase the polymer molecular weight and promote gelation.
Previously described compositions of keratin-based hydrogels have been reported. However, many of the hydrogels rely on relatively high protein percentages to establish and maintain the structure of the hydrogel. Accordingly, there is a great need to prepare compositions that form and maintain hydrogels at low protein percentages.